Prevention of distortion



Jan. 29, 1946. H. ROMANDER 2,393,936

PREVENTION OF DISTORTION Filed Feb. 5, 1942 v IN VENTOR.

- 4 060 imam/.054 BY A T TORNEY latent'ed Jan. 29, 1946 PREVENTION OF DISTORTION Hugo Romander, North Caldwell, N. J., assignor to Federal Telephone and Radio Corporation Application February 5, 1942, Serial No. 429,618

' 3 Claims. (01. 179-171) This invention relates to distortion correcting circuits and has for its object to correct or cancel distortion at the output of vacuum tube repeaters supplied with operating voltages from power supply sources.

' imperfect regulation" of the power supply voltage results in a varying voltage effectively applied to the repeater anode, the voltage variations corresponding with the frequency and wave form of the undulating voltage across the power source; with consequent impairment of the fidelity of the repeater. It is not usually practical to eliminate the distortion bymaking the impedance of the voltage supply small enough because this would require the use of a, capacity across the output of the source which would have to be so large as to be very expensive and occupy a large space, particularly when designed to withstand a large voltage.

In accordance with my invention the distortion is corrected without making the impedance of the voltage source small, by applying a voltage wave to the repeater input in the proper frequency, phase and amplitude to compensate for the voltage wave developed in the output due to the imperfect regulation of the power supply voltage. I The compensating wave is obtained by connecting to the power source a circuit including a transformer, the output of which is led to the input of the repeater, so that the distorting voltage wave developed across the power source produces at the input a counterpart of a phase and amplitude to cancel the distorting wave in the output circuit.

In the drawing,

Fig. 1 illustrates a part of a wave transmission system embodying this invention; and

Fig. 2 is a graphical representation of waves 1 ii ligpritssed on a repeater stage of the system in The system shown in Fig. 1 comprises an audio frequency repeater stage 13 in the well-known form of a push-pull amplifier comprising two tubes having the.respective cathodes I4 and I4, control grids l5 and I5, and anodes l8 and I3,- The input of stage I3 is supplied in a Well-known manner through a push-pull type of input transformer I 1 from some suitable source such as a preceding push-pull amplifying stage IB which receives the audio frequency waves on its input terminals l9 and 20 from some form of audio frequency system. The output of stage I3 is impressed across the primary winding 2! of a push-pull type of audio frequency output transformer 22, the secondary coil 23 of which is connected to a suitable load circuit. The repeater stag might be used to operateany desired type of load circuit, the particular type shown in the drawing being a radio frequency repeater stage I such as might be incorporated in a radio frequency transmitter. The arrangement shown is one in which the output of the audio sta e I3 is used to modulate the amplitude of the plate voltage applied to radio frequency stage I. The

radio frequency stage 1 comprises a vacuum tube having a cathode 2, a grid 3 and an anode 4, and is adjusted for class'C operation by negatively biasing the grid beyond the anode current cut-off point. A radio frequency wave is impressed on input terminals 5 and '6 of the stage from any suitable source (not. shown) and apat one side to a point 24 of the output circuit of radio frequency stage I, and at the other side through condensers 25 and 25 to ground; so that the audio frequency is impressed across condenser 9 of the radio frequency stage to produce modulation.

Power is supplied to the audio frequency stage IS in a well-known manner from a power supply system comprising a power transformer 21 having a primary coil 28 connected to a power source such asa cycle main at terminals 29 and 30. The secondary coil 3| is connected in a Wellknown manner in a series circuit from ground,

component, to the primary coil 2| of the pushpull stage l3 to supply the anode current therefor.

The stage I3 is of the class B type wherein the the rectified power supply. This condition is 11- lustrated graphically in Fig. 2 wherein the abscissa E8 represents the grid biasing voltage at control grids l5 and i5 and the ordinate I represents the anode current in the stage. The curve 39 represents the characteristic curve of anode currentvs. grid voltage for the stage. Thus, for

the condition of the class B amplifier assumedherein, the negative biasing voltage should be established at about the value Go for which the anode current is zero when no signal is present.

When an audio frequency wave is impressed on the input of the stage I3, the audio frequency wave w is impressed on the input of one tube, and a similar wave wi displaced 180 degrees out of phase from wave 20, is impressed on the input of the other tube. Only those parts of the input wave swings which extend in the positive direction from the biasing voltage 60 are received in the anode circuit, these being represented by the half waves W (due to input waves w) and W1 (due to input waves wi) The effect of the generation of these half waves or pulses in the anode circuit is to draw from the power supply, anode current in this wave form, the fundamental frequency of which is twice the frequency of the input signal wave. As the condenser 34 is of limited capacity, the eflect of drawin the heavypulses of current from it is to produce corresponding voltage drops across this condenser which has the effect of reducing the voltage applied to the tube anodes, in correspondence with these pulses. Such drops in anode voltage would produce distortion of the output wave form so that it does not truly represent the input wave.

In accordance with this invention, I haveprovided a circuit which corrects or cancels the distortion wave due to the poor regulation when condenser 34 is of modest capacity. This correcting circuit comprises an audio frequency transformer 40 having a primary coil 4| and asecondary coil 42, the primary coil being connected in series with a D. C. blocking condenser 43 (for example, 2 m. f.) and receiving A. C. voltage from across condenser 34. The secondary coil 42 is connected in series between the biasing voltage at 35 and the lead 38 to the control grids. Thus, in the secondary coil 42 there is generated a voltage corresponding to the audio frequency wave across condenser 34, and it is impressed on the input grids of the amplifier to produce their counterpart in the anode circuit,

The transformer 40 can readily be designed to produce the desired cancellation, from a knowledge of the characteristics of the tubes in stage l3 and the impedance of the power supply source used therewith. For example, although the wave form W, W1 shown in Fig. 2, which is; the form of the wave to be cancelled, is seen to. contain an infinite number of frequencies beginning with the lowest or fundamental frequency of twice the lowest signal frequency, the cancellation need be made only with respect to the lower frequencies of the entire range, because the impedance of the aaeacse capacity 34 will ordinarily be low enough to substantially short-circuit the higher frequencies. If, for example, the lowest signal frequency to be transmitted is about 30 cycles per second, the transformer 40 should be designed to transmit the frequency range from 60 cycles up to perhaps 1,000 cycles per second. The voltage for which the transformer should be designed will be about the voltage due to the fundamental frequency of the distortionwave, which will be the product of the impedance looking into the output of the voltage source and the fundamental frequency current, the latter being as is well-known, about 45% of the peak current to be used in the anode circuit. The voltage to be taken from the secondary coil 42 should be just sufficient that it will cancel out the distortion voltage at the output when sent through the stage.

I have found that by the use of such a circuit arrangement, the phase of the compensating or cancelling wave received at the output of the stage will be such as to cancel out the distorting wave. It is noted, however, that it is possible to connect the secondary coil 42 to the input circuit in the wrong phase, which would not produce cancellation; and if that is the case the polarity of the terminals of secondary coil 42 should be reversed to produce the proper phase relation in which the compensating voltage due to it opposes the distortion voltage in the anode circuit.

The attainment of good cancellation may be facilitated by the use of the adjustable connection 44 for varying the amount of correcting voltage used on the input. A distortion meter connected at the output of the stage can be used to show readily when the proper cancellation is had, as the adjustment would be proper when the output wave form corresponds with the input signal wave.

The invention is not necessarily limited to the particular arrangement shown. For example, it might be used even though the precise class B relationship did not exist in stage 13. For example, the cut-ofi voltage might be somewhat more or less than is shown in Fig. 2, and the advantage of the invention still obtained. Furthermore, the power supply system might be used to supply power to more than merely the corrected stage. It is shown supplying power through choke coils 45 and 46 to the anode 4 of the radio frequency stage I; and it could also be used for other tubes and voltages of the system. Moreover, a, stage with which the circuit of this invention is used need not necessarily supply signals to a modulator; it might be used to operate some other kind of apparatus, for example, a loudspeaker.

What is claimed is:

1. In combination, a push-pull vacuum tube repeater of the class B type having input and output electrodes, an alternating current supply source for supplying operating voltage to the output electrodes, said source having an appreciable impedance whereby there is produced at said source wave pulses due to a signal wave at the input electrodes, and a transformer having a primary and a secondary winding, the primary winding designed to transmit at least the lower frequency part of said wave pulse, being connected across at least part of said source and the secondary winding being connected to said input electrodes in the phase which cancels the distortion wave in the output circuit. 1

2. In combination, a vacuum tube repeater comprising two vacuum tubes, each having a cathode, a control electrode and an anode and connected in push-pull relation, said control electrodes having impressed thereon biasing voltage 01' the value which substantially reduces the anode current to zero, an alternating current supply source having across its output a capacity of appreciable impedance to at least the lower frequencies of the range to be transmitted by said repeater whereby a distortion wave of twice the fundamental frequency of a signal wave at the control electrodes is'produced at the anodes and source, and a transformer having a primary winding coupled across said condenser and a secondary winding coupled between said control electrodes and cathodes in the phase which opposes the distortion wave in the anode circuit.

3. A vacuum tube repeater system comprising a vacuum tube having input and output elecing in the output circuit voltage waves corresponding to wave cycles-impressed upon the input circuit, an alternating current supply source supplying operating voltage to the output anode through the output circuit, said source having an appreciable impedance suflicient to produce voltage fiuctuations in the output circuit during substantial current surges therein, a capacitance across said impedance, a transformer having a primary winding shunted across said capacitance and a secondary winding connected to said input circuit arranged to impress thereon a fluctuating voltage of the proper phase and amplitude to counterbalance said output circuit fluctuations, and means for varying the effective inductance of said transformer secondary winding.

HUGO ROMANDER. 

